The present invention relates generally to electroluminescent devices. More particularly, the present invention relates to outdoor electroluminescent display devices that are visible under a wide variety of ambient light conditions.
There are numerous applications for signs that can be read under conditions of both low and high ambient light (e.g., at night and in daylight). Road signs are one common example. Typically, these signs are printed so that they provide high contrast under reflected light. In daylight, they are readable due to reflected sunlight, and at night they are visible due to reflected artificial light, such as from the headlights of an automobile or from a dedicated spotlight aimed at the sign. Signs that are visible at night by means of a dedicated spotlight are inefficient from an energy standpoint, because most of the light from the spotlight is wasted in illuminating regions that do not require illumination for sign readability. Signs that rely on lights from automobile headlights from night-time readability have the disadvantage that they are only visible when the car is aiming directly at them. In addition, these signs can be hard to read at night in rainy conditions where much of the light from the car headlights is scattered by the rain droplets.
Cathode ray tube (CRT) technology has been developed to allow customizable signs that can be read in the dark. Such displays can be quickly reconfigured electronically to display a new sign. Other technologies with similar capabilities include liquid crystal displays, thin film plasma displays, and organic electroluminescent displays. However, these displays require complicated electronics and are too expensive for many simple display sign applications.
U.S. Pat. No. 5,962,962 describes a basic organic light emitting device (OLED). The OLED has a structure in which an anode, an organic light emitting layer, and a cathode are consecutively laminated, with the organic light emitting layer sandwiched between the anode and the cathode. Generally, electrical current flowing between the anode and cathode passes through points of the organic light emitting layer and causes it to luminesce. The electrode positioned on the surface through which light is emitted is formed of a transparent or semi-transparent film. The other electrode is formed of a specific thin metal film, which can be a metal or an alloy.
The organic light emitting device is usually formed on a substrate. In some cases, a hole-transporting layer is provided between the anode and the organic light emitting layer, and an electron-injecting layer is provided between the cathode and the organic light emitting layer, for improved performance.
Another example of an OLED is disclosed in U.S. Pat. No. 5,902,688. This patent discloses displays produced by providing an insulator that is patterned in a desired shape. In general, light will be produced in an OLED wherever an anode and cathode have a direct path to a particular point of the OLED. The insulator of this patent is in the form of an incomplete layer between the organic layer and one of the electrodes, and prevents contact of at least one electrode from a pattern of particular points on the OLED, which prevents those points of the organic layer from illuminating. To achieve a particular display, the insulator layer is patterned so as to retard the flow of current through the organic layer in proportion to the areas of the desired display that are to be relatively dark. However, a sign using this display depends only on the contrast between the illuminated and dark areas of the device, and the luminance that is possible with practical OLED devices is relatively low ( less than 10,000 candelas per square meter (Cd/m2)). Because of this, while these displays may be visible during conditions of low light, they cannot be read under bright ambient conditions (e.g., in the midday sun). Further, the process of forming the insulating layer is difficult and time-consuming, which increases the cost of production of such a device.
It would be desirable to provide a cost-effective sign that is readable under low and high ambient light conditions, which does not suffer from the above disadvantages. It would further be desirable to provide a simple method for production of such a sign.
A device structure and method are disclosed that enable the production of outdoor signs utilizing organic light emitting device (OLED) technology. Exemplary embodiments of the device include an OLED that is patterned into a sign combined with an exterior layer comprising (1) a highly scattering, non-absorbing coating over the OLED emitting regions, and (2) a highly absorbing coating over the non-emitting regions. The result is a sign that can be viewed using organic electroluminescent light under low ambient light conditions, and due to the exterior coating, can also be viewed under high ambient light conditions. Thus, embodiments of the present invention provide a degree of contrast, both in natural daylight and at night, which facilitates easy viewing of, for example, a street sign.